73 research outputs found
Flavor decomposition of the sea quark helicity distributions in the nucleon from semi-inclusive deep-inelastic scattering
Double-spin asymmetries of semi-inclusive cross sections for the production
of identified pions and kaons have been measured in deep-inelastic scattering
of polarized positrons on a polarized deuterium target. Five helicity
distributions including those for three sea quark flavors were extracted from
these data together with re-analyzed previous data for identified pions from a
hydrogen target. These distributions are consistent with zero for all three sea
flavors. A recently predicted flavor asymmetry in the polarization of the light
quark sea appears to be disfavored by the data.Comment: 5 pages, 3 figure
Evidence for Quark-Hadron Duality in the Proton Spin Asymmetry
Spin-dependent lepton-nucleon scattering data have been used to investigate
the validity of the concept of quark-hadron duality for the spin asymmetry
. Longitudinally polarised positrons were scattered off a longitudinally
polarised hydrogen target for values of between 1.2 and 12 GeV and
values of between 1 and 4 GeV. The average double-spin asymmetry in
the nucleon resonance region is found to agree with that measured in
deep-inelastic scattering at the same values of the Bjorken scaling variable
. This finding implies that the description of in terms of quark
degrees of freedom is valid also in the nucleon resonance region for values of
above 1.6 GeV.Comment: 5 pages, 1 eps figure, table added, new references added, in print in
Phys. Rev. Let
The Q^2-Dependence of Nuclear Transparency for Exclusive Production
Exclusive coherent and incoherent electroproduction of the meson
from H and N targets has been studied at the HERMES experiment as a
function of coherence length (), corresponding to the lifetime of hadronic
fluctuations of the virtual photon, and squared four-momentum of the virtual
photon (). The ratio of N to H cross sections per nucleon,
known as nuclear transparency, was found to increase (decrease) with increasing
coherence length for coherent (incoherent) electroproduction. For
fixed coherence length, a rise of nuclear transparency with is observed
for both coherent and incoherent production, which is in agreement
with theoretical calculations of color transparency.Comment: 5 pages, 4 figure
First Measurement of the Tensor Structure Function of the Deuteron
The \Hermes experiment has investigated the tensor spin structure of the
deuteron using the 27.6 GeV/c positron beam of \Hera. The use of a tensor
polarized deuteron gas target with only a negligible residual vector
polarization enabled the first measurement of the tensor asymmetry \At and
the tensor structure function \bd for average values of the Bj{\o}rken
variable and of the squared four-momentum transfer . The quantities \At and \bd are found to be
non-zero. The rise of \bd for decreasing values of can be interpreted to
originate from the same mechanism that leads to nuclear shadowing in
unpolarized scattering
The -dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution
to the magnetic moment of the target nucleus with an energy-weighted integral
of the difference of the helicity-dependent photoabsorption cross sections. The
data collected by HERMES with a deuterium target are presented together with a
re-analysis of previous measurements on the proton. This provides a measurement
of the generalised GDH integral covering simultaneously the nucleon-resonance
and the deep inelastic scattering regions. The contribution of the
nucleon-resonance region is seen to decrease rapidly with increasing . The
DIS contribution is sizeable over the full measured range, even down to the
lowest measured . As expected, at higher the data are found to be in
agreement with previous measurements of the first moment of . From data on
the deuteron and proton, the GDH integral for the neutron has been derived and
the proton--neutron difference evaluated. This difference is found to satisfy
the fundamental Bjorken sum rule at GeV.Comment: 12 pages, 10 figure
Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target
Single-spin asymmetries for semi-inclusive electroproduction of charged pions
in deep-inelastic scattering of positrons are measured for the first time with
transverse target polarization. The asymmetry depends on the azimuthal angles
of both the pion () and the target spin axis () about the virtual
photon direction and relative to the lepton scattering plane. The extracted
Fourier component \cmpi is a signal of the previously unmeasured quark
transversity distribution, in conjunction with the so-called Collins
fragmentation function, also unknown. The Fourier component \smpi of the
asymmetry arises from a correlation between the transverse polarization of the
target nucleon and the intrinsic transverse momentum of quarks, as represented
by the previously unmeasured Sivers distribution function. Evidence for both
signals is observed, but the Sivers asymmetry may be affected by exclusive
vector meson productio
Search for an exotic S=-2, Q=-2 baryon resonance at a mass near 1862 MeV in quasi-real photoproduction
A search for an exotic baryon resonance with has been performed
in quasi-real photoproduction on a deuterium target through the decay channel
. No evidence for
a previously reported resonance is found in the invariant mass spectrum. An upper limit for the photoproduction cross
section of 2.1 nb is found at the 90% confidence level. The photoproduction
cross section for the is found to be between 9 and 24 nb
Single-spin Azimuthal Asymmetries in Electroproduction of Neutral Pions in Semi-inclusive Deep-inelastic Scattering
A single-spin asymmetry in the azimuthal distribution of neutral pions
relative to the lepton scattering plane has been measured for the first time in
deep-inelastic scattering of positrons off longitudinally polarized protons.
The analysing power in the sin(phi) moment of the cross section is 0.019 +/-
0.007(stat.) +/- 0.003(syst.). This result is compared to single-spin
asymmetries for charged pion production measured in the same kinematic range.
The pi^0 asymmetry is of the same size as the pi^+ asymmetry and shows a
similar dependence on the relevant kinematic variables. The asymmetry is
described by a phenomenological calculation based on a fragmentation function
that represents sensitivity to the transverse polarization of the struck quark.Comment: 4 pages, 1 figure, replaced to correct eprint author field, 2nd
replacement to correct figure; upper limit of model predictions are
corrected. No correction to data or conclusion
Double hadron leptoproduction in the nuclear medium
First measurement of double-hadron production in deep-inelastic scattering
has been measured with the HERMES spectrometer at HERA using a 27.6 GeV
positron beam with deuterium, nitrogen, krypton and xenon targets. The
influence of the nuclear medium on the ratio of double-hadron to single-hadron
yields has been investigated. Nuclear effects are clearly observed but with
substantially smaller magnitude and reduced -dependence compared to
previously measured single-hadron multiplicity ratios. The data are in fair
agreement with models based on partonic or pre-hadronic energy loss, while they
seem to rule out a pure absorptive treatment of the final state interactions.
Thus, the double-hadron ratio provides an additional tool for studying
modifications of hadronization in nuclear matter
Measurement of single-spin azimuthal asymmetries in semi-inclusive electroproduction of pions and kaons on a longitudinally polarised deuterium target
Single-spin asymmetries have been measured for semi-inclusive
electroproduction of , , and mesons in
deep-inelastic scattering off a longitudinally polarised deuterium target. The
asymmetries appear in the distribution of the hadrons in the azimuthal angle
around the virtual photon direction, relative to the lepton scattering
plane. The corresponding analysing powers in the moment of the
cross section are for ,
for ,
for and for . The moments are
compatible with zero for all particles.Comment: Revised version shortened 9 pages, 3 tables, 7 figure
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